Rotomoulded articles prepared with polyethylene

ABSTRACT

The present invention is concerned with rotomoulded articles having very low warpage and shrinkage and consisting essentially of polyethylene prepared with a catalyst system based on a bis-indenyl or on a bis(n-butylcyclopentadienyl) metallocene component.

This invention is concerned with rotomoulded articles having reducedshrinkage and warpage and prepared from polyethylene polymerised with atetrahydro-indenyl catalyst.

Polyethylene represents more than 80% of the polymers used in therotomoulding market. This is due to the outstanding resistance ofpolyethylene to thermal degradation during processing, to its easygrinding, good flowability, and low temperature impact properties.

Rotomoulding is used for the manufacture of simple to complex, hollowplastic products. It can be used to mould a variety of materials such aspolyethylene, polypropylene, polycarbonate or PVC. Linear low densitypolyethylene is preferably used as disclosed for example in “Some newresults on rotational molding of metallocene polyethyles” by D.Annechini, E. Takacs and J. Vlachopoulos in ANTEC, vol. 1, 2001.

Polyethylenes prepared with a Ziegler-Natta catalyst are generally usedin rotomoulding, but metallocene-produced polyethylenes are desirable,because their narrow molecular distribution allows better impactproperties and shorter cycle time in processing.

The metallocene-produced polyethylenes of the prior art (see ANTEC, vol.1, 2001) suffer from high shrinkage and warpage.

Plastoelastomeric compositions such as described in U.S. Pat. No.5,457,159 can also be used in rotomoulding, but they require complexprocessing steps of mixing and vulcanisation.

U.S. Pat. No. 6,124,400 discloses the use for rotomoulding of polymeralloys containing semi-crystalline polyolefin sequences with chains ofdifferent controlled microstructure prepared in a “one-pot”polymerisation process from a single monomer. The polymerization ofthese polymer alloys requires a complex catalyst system comprisingorganometallic catalyst precursors, cationic forming cocatalysts andcross-over agents.

There is thus a need for rotomoulded articles that do not suffer fromthese drawbacks.

It is an aim of the present invention to prepare rotomoulded articleswith low shrinkage.

It is also an aim of the present invention to produce rotomouldedarticles with very little warpage.

It is another aim of the present invention to prepare rotomouldedarticles having impact strength and ease of processing.

It is a further aim of the present invention to prepare rotomouldedarticles with a very short cycle time.

It is yet another aim of the present invention to prepare rotomouldedarticles having a very fine microstructure.

It is yet a further aim of the present invention to prepare rotomouldedarticles having an excellent gloss.

Accordingly, the present invention discloses articles produced byrotomoulding and consisting essentially of polyethylene (PE) polymerizedwith a metallocene catalyst system based on a bis-indenyl or on abis-cyclopentadienyl metallocene catalyst component.

The high density polyethylene used in the present invention has adensity ranking from 0.915 to 0.950 g/cm³, preferably from 0.925 to0.945 g/cm³ and a melt flow index of from 0.5 to 30 g/10 min, preferablyfrom 2.0 to 20 g/10 min.

In this specification, the density of the polyethylene is measured at23° C. using the procedures of standard test ASTM D 1505.

The melt index M12 is measured using the procedures of standard testASTM D 1238 at 190° C. and under a load of 2.16 kg.

The metallocene used to prepare the high density polyethylene can be abis-indenyl represented by the general formula:R″(Ind)₂ MQ₂   (I)or a bis-cyclopentadienyll represented by the formula(Cp)₂ MQ₂   (II)wherein (Ind) is an indenyl or an hydrogenated indenyl, substituted orunsubstituted, Cp is a cyclopentadienyl ring substituted orunsubstituted, R″ is a structural bridge between the two indenyls toimpart stereorigidity that comprises a C₁-C₄ alkylene radical, a dialkylgermanium or silicon or siloxane, or a alkyl phosphine or amine radical,which bridge is substituted or unsubstituted; Q is a hydrocarbyl radicalhaving from 1 to20 carbon atoms or a halogen, and M is a group IVbtransition metal or Vanadium.

In formula (I), each indenyl or hydrogenated indenyl compound may besubstituted in the same way or differently from one another at one ormore positions in the cyclopentadienyl ring, the cyclohexenyl ring andthe bridge.

In formula (I), each substituent on the indenyl may be independentlychosen from those of formula XR_(v) in which X is chosen from group IVA,oxygen and nitrogen and each R is the same or different and chosen fromhydrogen or hydrocarbyl of from 1 to 20 carbon atoms and v+1 is thevalence of X. X is preferably C. If the cyclopentadienyl ring issubstituted, its substituent groups must be so bulky as to affectcoordination of the olefin monomer to the metal M. Substituents on thecyclopentadienyl ring preferably have R as hydrogen or CH₃. Morepreferably, at least one and most preferably both cyclopentadienyl ringsare unsubstituted.

In a particularly preferred embodiment, both indenyls are unsubstituted.

In formula (II), each cyclopentadienyl ring may be substituted in thesame way or differently from one another at one or more positions in thecyclopentadienyl ring.

In formula (II), each substituent on the cyclopentadienyl may beindependently chosen from those of formula XR*_(v) in which X is chosenfrom group IVA, oxygen and nitrogen and each R* is the same or dfferentand chosen from hydrogen or hydrocarbyl of from 1 to 20 carbon atoms andv+1 is the valence of X. X is preferably C and the most preferredsubstituent is n-butyl.

R″ is preferably a C1-C4 alkylene radical (as used herein to describe adifunctional radical, also called alkylidene), most preferably anethylene bridge (as used herein to describe a difunctional radical, alsocalled ethylidene), which is substituted or unsubstituted.

The metal M is preferably zirconium, hafnium, or titanium, mostpreferably zirconium.

Each Q is the same or different and may be a hydrocarbyl or hydrocarboxyradical having 1 to 20 carbon atoms or a halogen. Suitable hydrocarbylsinclude aryl, alkyl,alkenyl,alkylaryl or arylalkyl. Each Q is preferablyhalogen.

Among the preferred metallocenes used in the present invention, one cancite bis tetrahydro-indenyl compounds and bis indenyl compounds asdisclosed for example in WO 96/35729 or bis(cyclopentadienyl) compounds.The most preferred metallocene catalysts are ethylene bis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride andbis(n-butyl-cyclopentadienyl) zirconium dichloride.

The metallocene may be supported according to any method known in theart. In the event it is supported, the support used in the presentinvention can be any organic or inorganic solids, particularly poroussupports such as talc, inorganic oxides, and resinous support materialsuch as polyolefin. Preferably, the support material is an inorganicoxide in its finely divided form.

The addition on the support, of an agent that reacts with the supportand has an ionising action, creates an active site.

Preferably, alumoxane is used to ionise the catalyst during thepolymerization procedure, and any alumoxane known in the art issuitable.

The preferred alumoxanes comprise oligomeric linear and/or cyclic alkylalumoxanes represented by the formula:

for oligomeric, linear alumoxanes And

for oligomeric, cyclic alumoxanes, wherein n is 140, preferably 10-20, mis 3-40, preferably 3-20 and R is a C₁-C₈ alkyl group and preferablymethyl. Methylalumoxane is preferably used.

One or more aluminiumalkyl(s) can be used as cocatalyst in the reactor.An aluminiumalkyl represented by the formula AlR₃ can be used whereineach R is the same or different and is selected from halides or fromalkoxy or alkyl groups having from 1 to 12 carbon atoms. Especiallysuitable aluminiumalkyl is trialkylaluminium, the most preferred beingtriisobutylaluminium (TIBAL).

Further, the catalyst may be prepolymerised prior to introducing it inthe reaction zone and/or prior to the stabilization of the reactionconditions in the reactor.

The polymerisation of the metallocene-produced high density polyethylenecan be carried out in gas, solution or slurry phase. Slurrypolymerisation is preferably used to prepare the high densitypolyethylene. The polymerisation temperature ranges from 20 to 125° C.,preferably from 60 to 95° C. and the pressure ranges from 0.1 to 5.6Mpa, preferably from 2 to 4 Mpa, for a time ranging from 10 minutes to 4hours, preferably from 1 and 2.5 hours.

A continuous single loop reactor is preferably used for conducting thepolymerisation under quasi steady state conditions. A double loopreactor can also be used to produce either monomodal or bimodal resins,such as for examples a resin consisting of a first fraction produced inthe first reactor under first polymerisation conditions and a secondfraction produced in the second reactor under second polymerisationconditions, said two fractions having the same molecular weight anddifferent densities.

The average molecular weight is controlled by adding hydrogen duringpolymerisation. The relative amounts of hydrogen and olefin introducedinto the polymerisation reactor are from 0.001 to 15 mole percenthydrogen and from 99.999 to 85 mole percent olefin based on totalhydrogen and olefin present, preferably from 0.2 to 3 mole percenthydrogen and from 99.8 to 97 mole percent olefin.

The density of the polyethylene is regulated by the amount of comonomerinjected into the reactor; examples of comonomer which can be usedinclude 1-olefins, typically C3 to C20 olefins among which propylene,butene, hexene, octene, 4-methyl-pentene are preferred, the mostpreferred being hexene.

The rotomoulding machine can be any one of the machines generally usedin the field such as for example the CACCIA 1400R rotational mouldingmachine.

The rotomoulded polyethylene articles according to the present inventionare characterised by very low warpage and shrinkage.

The polyethylene structure is mainly influenced by the catalytic systemused for polymerisation and said structure is responsible for theproperties of the final articles. It has been observed that a n-butylcatalyst produces a linear polyethylene resin with a narrow molecularweight distribution of about 2.5, that a Ziegler-Natta catalyst producesa linear polyethylene resin with a broader molecular weight distributionof the order of 5 and that a tetrahydro-indenyl catalyst produces apolyethylene with a large amount of long chain branches and a narrowmolecular weight distribution of the order of 2.5.

The molecular weight distribution (MWD) is completely defined by thepolydispersity index D that is the ratio Mw/Mn of the weight averagemolecular weight (Mw) to the number average molecular weight (Mn).

The Dow Rheological Index (DRI) gives a measure of the amount of longchain branches. The lower the DRI value, the lower the amount of longchain branches. In the present invention the DRI is determined byfitting the Rheological Dynamic Analysis (RDA) curve of the HDPE by theCross rheological model described here-below.

The dynamic rheology is measured using the method of the RDA. It is ameasure of the resistance to flow of material placed between twoparallel plates rotating with respect to each other with an oscillatorymotion. The apparatus comprises a motor that transmits a sinusoidaldeformation to the sample. The sample then transmits the resultingconstraint, said resulting constraint being also sinusoidal. Thematerial to be studied can be a solid attached between two anchoringpoints or it can be melted between the two plates. The dynamic rheometerallows the simultaneous measurement of both the elastic modulus and theviscous modulus of the material. Indeed, the resulting sinusoidalconstraint is displaced by a phase angle δ with respect to the imposeddeformation and it is mathematically possible to decompose the resultingsinusoid into:

-   a first sinusoid in phase with the initial deformation that    represents the elastic component of the material. Said component    conserves energy.-   a second sinusoid displaced by a phase angle of π/2 with respect to    the initial deformation that represents the viscous component. Said    component dissipates energy into heat.    The initial deformation is represented by the formula    γ=γ₀ sin (ωt)    wherein ω is the frequency.    The resulting constraint is thus of the form    τ=τ₀ sin (ωt+δ)    The complex modulus is given by the formula    G=τ/γ

The complex modulus can be decomposed into the elastic modulus G′ andthe viscous modulus G″ defined respectively asG′=G cos (δ)andG″=G sin(δ)

The complex viscosity is defined as G/ω. At constant temperature andconstant deformation amplitude, G′ and G″ can be measured for differentvalues of ω. The measurements were carried out under the followingoperating conditions:

-   a constant operating temperature of 190° C.,-   parallel plates separated by 1.5 mm,-   maximum deformation maintained at 10%.

The elastic component G′ and the viscous component G″ can be graphed asa function of frequency ω. The point of intersection between the elasticand viscous curves, called the cross-over point (COP), is characterisedby a frequency ω_(c) and a viscosity component G_(c). The cross-overpoint is characteristic of each polymer and is a function of themolecular weight and of the molecular distribution.

To characterize the Theological behavior of substantially linearethylene polymers, S. Lai and G. W. Knight have introduced ( ANTEC '93Proceedings, lnsite™ Technology Polyolefins (ITP)—New Rules in theStructure/Rheology Relationship of Ethylene &-Olefin Copolymers, NewOrleans, La., May 1993) a new rheological measurement, the Dow RheologyIndex (DRI) which expresses a polymer's “normalized relaxation time asthe result of long chain branching”. S.Lai et al; ( Antec '94, DowRheology Index (DRI) for lnsite™ Technology Polyolefins (ITP): Uniquestructure-Processing Relationships, pp. 1814-1815) defined the DRI asthe extent to which the rheology of ethylene- octene copolymers known asITP ( Dow's Insite Technology Polyolefins) incorporating long chainbranches into the polymer backbone deviates from the rheology of theconventional linear homogeneous polyolefins that are reported to have noLong Chain Branches (LCB) by the following normalized equation:DRI=(365000. λ/η₀−1)/10wherein λ is the characteristic relaxation time of the material and η₀is the zero shear viscosity of the material. The DRI is calculated byleast squares fit of the rheological curve (complex viscosity versusfrequency) as described in U.S. Pat. No. 6,114,486 with the followinggeneralized Cross equation, i.e.η=η₀/(1+(γ λ)^(n))wherein n is the power law index of the material, η and γ are themeasured viscosity and shear rate data respectively. The dynamicrheological analysis was performed at 190° C. and the strain amplitudewas 10%. Results are reported according to ASTM D 4440. A low value ofthe Dow rheological index is indicative of low or inexistant Long ChainBranching (LCB). At equivalent molecular weight distribution, thecontent of LCB increases with increasing DRI. A value of DRI above oneindicates a high level of LCB. It is also known that a high level of LCBis associated with a large elastic component as indicated by dynamicrheology.

The resins according to the present invention further have excellentflexural yield strength and flexural properties. They also have goodimpact strength both at room temperature and at low temperatureAdditionally, the production cycling time of the polyethylene resinsproduced with a bis-indenyl catalyst and preferably with atetrahydro-indenyl catalyst is in line with that of other polyethyleneresins.

It has further been observed that the rotomoulded articles prepared withthe polyethylene according to the present invention offer a betterresistance to degradation from nitric acid.

In yet another embodiment of the present invention, the rheologicalproperties of the polyethylene produced with a tetrahydro-indenylcatalyst are used to prepare micro-pellets having an average size offrom 300 to 800 microns in a one-step procedure: it is a result the fastdecrease of the viscosity with increasing shear.

In yet a further embodiment of the present invention, rotomouldedarticles having an inner foamed polyethylene layer and an outer normalpolyethylene layer can be prepared. The large elastic viscositycomponent G′ of the polyethylene prepared with a tetrahydro-indenylcatalyst is responsible for the better dispersion of bubbles into thefoamed material.

In another aspect of the present invention, the articles prepared with acatalyst component based either on tetrahydro indenyl or onbis(n-butyl-cyclopentadienyl) have a very fine microstructure, therebyimproving their mechanical properties such as impact strength and theiroptical properties, such as gloss and their impermeability to solvents.

The polyethylene resins of the present invention can also becross-linked by any cross-linking agent known in the field prior tobeing rotomoulded.

The polyethylene polymerized with a bis-indenyl or abis)cyclopentadienyl metallocene catalyst according to the presentinvention can be used to produce rotomoulded articles in a variety ofapplications such as for example tanks, containers, toys, boats,furniture, medical applications, buried tanks, sceptic tanks, fueltanks.

LIST OF FIGURES

FIG. 1 represents the viscous modulus G″ expressed in Pa as a functionof the elastic modulus G′ expressed in Pa.

FIG. 2 represents the elastic component G′ expressed in Pa as a functionof shear rate expressed in s-1.

FIG. 3 represents containers produced respectively with resin R3 (FIG. 3a) anfdwith resin R9 (FIG. 3 b).

FIG. 4 represents the microstructure of resins R9 (FIG. 4 a), R1 (FIG. 4b), R10 (FIG. 4 c) and R3 (FIG. 4 d).

FIG. 5 is a graph representing the average permeability to fuelexpressed in g/day at 40° C. for rotomoulded boffles prepared withresins R9, R10, R1 and R3.

FIG. 6 represents the peak internal air temperature (PIAT) temperatureexpressed in ° C. for a normal mould as a function of time expressed inseconds.

FIG. 7 represents the peak internal air temperature (PIAT) temperatureexpressed in ° C. for a pressurised mould as a function of timeexpressed in seconds.

FIG. 8 represents the radius X/a as a function of time expressed inseconds in sintering experiments, wherein X is the sintering neck radiusbetween the two spheres to be sintered and a is the radius of thespheres.

FIG. 9 represents the mass fraction of the powder expressed in percentas a function of particle size expressed in microns.

FIG. 10 represents the number of bubbles per mm² remaining in the moltenresin as a function of temperature expressed in ° C.

FIG. 11 represents the mass fraction of the micropellets expressed inpercent as a function of particle size expressed in microns.

FIG. 12 represents a photograph of the micropellets produced on machine2 for resin R9, under the conditions described in Table VII.

FIG. 13 represents the mass fraction of powders and micropelletsexpressed in percent as a function of particle size expressed in micronsfor resin R7.

FIG. 14 represents the number of bubbles per mm² remaining in the moltenresin as a function of temperature expressed in ° C. for several powderand micropellet samples of various granulometries.

EXAMPLES

Several high density polyethylene resins have been tested and comparedin rotomoulding applications.

Resin R1 is a polyethylene resin prepared with a metallocene catalystand sold by Borealis under the name Borocene® RM8343.

Resins R2, R5 and R6 have been prepared with ethylenebis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride.

Resin R3 is a commercial Ziegler-Natta resin sold by BP under the nameRigidex® 3560

Resins R7 and R8 were prepared with di(n-butyl-cyclopentadieny)zirconium dichloride.

Resin R9 was prepared with ethylene bis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride.

Resin R10 was prepared with di(n-butyl-cyclopentadieny) zirconiumdichloride.

Resins R11 and R12 were prepared with ethylenebis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride and had a verylow melt index.

The resins' properties are summarised in Table I. TABLE I DRI @ MI2Density Resin η_(O) (Pa.s) catalyst 190° C. (g/10 min) (g/cm³) MWD R11045 metallocene 0.04 6 0.934 2.1 R2 3394.39 THI 3.88 4.2 0.939 2.9 R31357.83 ZN 0.37 6.38 0.936 4.0 R5 1518.88 THI 0.99 7.04 0.936 2.5 R61227.78 THI 0.63 7.9 0.937 2.3 R7 1106.25 n-butyl 0.04 6.24 0.936 2.2 R8238.04 n-butyl 0.04 5.3 0.936 2.1 R9 950.00 THI 1 8 0.935 2.2  R101000.00 n-butyl 0 6 0.935 2.2  R11 >10000 THI 10.2 0.9 0.934 2.3 R12 >10000 THI 11.3 0.7 0.947 2.4Wherein η₀ is the zero-shear viscosity.

As can be seen from Table 1, the main difference between resins R1, R7,R8 and R10 on the one hand and resins R2, R5, R6 and R9 on the otherhand lies in the values for the Dow Rheological index (DRI), said valuesbeing extremely low for the resins prepared with the n-butyl catalyst.It must be noted that for resin R3 prepared with a Ziegler-Nattacatalyst the DRI value is influenced by the molecular weightdistribution that is broader than that of a resin prepared with ametallocene catalyst. It must also be noted that resins R11 and R12 havemelt flow indices of 0.9 and 0.7 g/10 min, well below the minimum valueof about 2 g/10 min generally recommended in the field of rotomoulding.

The elastic component of the various resins is displayed in FIG. 1 thatrepresents the viscous modulus G″ expressed in Pa graphed as a functionof the elastic modulus G′ expressed in Pa. It can be deduced from thatgragh that resins R2 and R3 prepared respectively with atetrahydro-indenyl catalyst and with a Ziegler-Natta catalyst have thelargest elastic component, resin R6, prepared with a tetrahydro-indenylcatalyst has an intermediate elastic component and resin R7 preparedwith the n-butyl catalyst has the lowest elastic component. R2 presentsa higher elastic component G′ than R6 because it has a higher molecularweight than R6.

FIG. 2 represents the elastic component G′ expressed in Pa as a functionof shear rate expressed in s-1. It is observed on that graph that resinR7 prepared with a n-butyl metallocene catalyst has the lowest elasticcomponent which is favourable to sintering. These rotomoulded articlesprepared from these resins however suffer from high shrinkage andwarpage. Resin R3, prepared with a Ziegler-Natta catalyst has a higherelastic component than R7, thus less favourable for sintering. Resin R6prepared according to the present invention has an intermediate positionbetween resins R7 and R3 and offers the advantage of very smallshrinkage and warpage.

A first set of rotational moulding trials were carried out on thepolyethylene powders, R2, R3, R6 and R7 micronised on the WEDCO®machine. Each grade polymer was exposed to three peak internal airtemperatures (PIAT) that were respectively of 180, 190 and 210° C. ThePIAT is defined as the maximum temperature that the mould reached withinthe oven before it was removed to the cooling bay of the rotationalmoulding machine. In all cases, the parts were moulded using an oven settemperature of 300° C. The ROTOLOG® temperature measuring system wasused to record the temperature profiles of the internal air, material,and mould as well as that of the oven. The system consists of aninsulated radio transmitter, which is attached to the mould and travelswith it in the oven and in the cooler bay.

The transmitter sends a signal to a receiver, which in turn is connectedto a computer that uses the ROTOLOG® software to graph real-timetemperature/time data.

All test mouldings were carried out on the CACCIA 1400R rotationalmoulding machine having the following specifications:

-   -   Shuttle-style machine    -   Offset arm    -   LPG burner arm    -   Burner capacity of 7700 Kcal/hr    -   Air fan cooling    -   Maximum plate diameter of 950 mm.

The mould used to produce the test mouldings was an aluminium cube mouldof base 300 mm×300 mm with a central vent port and with a draft angle of3° included to facilitate demoulding. The shot weight was set at 1.8 kgto produce 3 mm thick mouldings. The mould was removed from the oven atpeak internal temperatures of 180° C., 190° C. and 210° C. The coolingmedium for all the materials was forced air and the moulding conditionsfor the trials were as follows:

-   -   oven temperatue: 300° C.    -   rotation ratio: ˜4:1    -   cooling medium: forced air    -   preheated arm and mould    -   rotolog unit n° 5 and rotolog software version 2.7

The total cycle times for the mouldings are shown in Table II. All cycletimes are taken from the same start point of 50° C. and a demouldingtemperature of 79° C. to allow for easier comparison. TABLE II Overallcycle time in minutes Material PIAT of 180° C. PIAT of 190° C. PIAT of210° C. R2 33.05 33.47 36.27 R3 29.5 30.52 37.3 R6 30.93 32.99 38.3 R739.2 39.4 43.1

It can be seen from Table II that for all samples tested the overallcycle time increases with increasing PIAT.

Shrinkage and warpage were measured on all samples as follows.

Mould shrinkage factors are measured by recording how much a mouldedarticle dimension reduces after the moulding has cooled. The reduceddimension is related to a reference dimension taken from the actualmould. In the case of the box mould used in the present invention, themould had a grid machined into the bottom of its cavity. The distanceselected as the reference value was the hypothenuse distance of the gridat the bottom of the mould: it was measured to be 169.9 mm. The distancebetween the same two reference points was recorded on the cooledmoulding and the percentage of shrinkage was then determined. Themeasuring apparatus consisted of a milling machine bed upon which themoulded article was placed. An electronic microscope was fixed ontomovable axes positioned above the milling bed. Any movements of theaxes, and subsequently of the microscope, were measured on an electronicmeter so that the X and Y coordinates of movement could be obtained.Once the moulded article was positioned properly on the milling bed, theX and Y distances of the grid reproduced on the moulding could bemeasured and the diagonal distance between the selected grid pointscould be calculated and compared to the reference value of 169.9 mm,thereby allowing to calculate the percentage of shrinkage. The resultsare displayed in Table III TABLE III Shrinkage in % Material PIAT of180° C. PIAT of 190° C. PIAT of 210° C. R2 2.55 2.39 2.65 R3 2.43 2.672.83 R6 2.24 2.16 2.29 R7 3.28 3.12 3.07

The best results have been obtained with resin R6.

The amount of warpage on a moulded article was measured by using a dialgauge in conjonction with the apparatus described here-above formeasuring the shrinkage. The dial gauge pointer was placed above thecentre of the grid and the milling machine bed was raised vertically sothat a datum value could be set on the gauge. The milling bed was thenmoved so that the dial gauge sat on a point of the grid and a readingwas made of how much the pointer rose or fell with respect to the datumvalue. This was repeated for all the points on the grid and the maximumwarpage was defined as the largest deviation from the datum. The resultsfor warpage are displayed in Table IV. TABLE IV Maximum warpage in mmMaterial PIAT of 180° C. PIAT of 190° C. PIAT of 210° C. R1 2.45 2.21.41 R2 1.9 2.0 1.52 R3 2.45 3.25 1.72 R6 2.24 1.55 2.12 R7 3.28 2.62.95 R9 0.69 0.31 0.92  R10 2.6 2.6 2.9

It can be seen from the results displayed in Tables III and IV that theresin prepared with the n-butyl catalyst exhibited the largest valuesfor both shrinkage and warpage.

Impact measurements were carried out following the method of standardtest ASTM 5420 both at room temperature and at a temperature of 40° C.The test results were obtained on an average of at least 5 samples.

Modes of failure during impact testing fall into two categories: brittleand ductile. With brittle failure, a crack initiates and propagatesprior to any bulk yielding of the specimen and hence the point offailure lies on the initial rising portion of the load/deformationcurve. In the case of ductile failure, considerable yielding takes placeand the failure occurs well after the maximum on the load/deformationcurve. As the area under the load/deformation curve is a measure of thefracture energy, it follows that brittle failure is associated with verylow absorbed energy as compared to ductile failure.

The samples used for impact tests were all taken from the same side ofeach trial moulding so that the results were made comparable to themoulding conditions. They were cut with a bandsaw into squares of 60mm×60 mm, the edges were cleaned of burrs and the thickness at thecentre of each sample was noted. The machine used was the CEASTFractovis and depending upon the thickness and the expected strength ofthe sample under test, the sensitivity and the working range of the loadcell were appropriately set up to sense the sample failure.

The impact results were recorded both at room temperature and at ˜40° C.for resins R2, R3, R6 and R7 and respectively for the peak internal airtemperatures of 180, 190 and 210° C. The results are displayed in TableV. They are expressed in J/mm. TABLE V PIAT of 180° C. PIAT of 190° C.PIAT of 210° C. Resin T = −40° C. Room T T = −40° C. Room T T = −40° C.Room T R2 8.84 7.71 8.52 6.99 8.57 7.32 ductile ductile ductile/brittleductile ductile ductilt R3 8.83 7.08 6.58 5.5 10.3 7.69 ductile ductileductile/brittle ductile ductile/brittle ductile R6 8.09 7.01 7.2 7.297.02 7.12 brittle ductile brittle ductile brittle ductile R7 9.98 8.3410.36 7.82 10.25 8.56 brittle ductile ductile ductile ductile ductile

As can be observed, all the samples exhibited a ductile failure at roomtemperature. It is also observed that the cold temperature (−40° C.)impact values are higher than the room temperature impact values.

Stacking tests with nitric acid have also been performed on resins R2,R3 and R6. The measurements were carried out following the method of ADRStandard—Appendix A.5. The load applied had a density of 1.4 g/cm³ andthe containers were hollow 30 litres items of 1.8 kg. The times tofailure were respectively of 35 days for comparative ZN resin R3 and of150 days for resins R2 and R6 according to the present invention.

Surface gloss evaluations have been carried out on resins R1, R3, R9 andR10 using a standard finish mould and a mirror polished mould. The PIATwas 190° C. and the gloss was measured using the method of standard testASTM D 2547-90. The gloss results expressed in % are displayed in TableVI and in FIG. 3. TABLE VI 45° light incidence 20° light incidenceStandard Mirror Standard Mirror Resin finish (%) polished (%) finish (%)polished (%) R1 13 35 45 R3 12 43 48 R9 14 60 78  R10 13 47 60

It can be seen that the containers prepared with the resins of theinvention have a much better gloss than the reference resins usuallyused in rotomoulding applications.

The microstructure of resins R9 and R10 according to the invention andof reference resins R1 and R3 was also studied following the methoddescribed in Oliveira and Cramez (Oliveira M. J., and Cramez M. C.; J.Macrom. Sci.-Physics, B40, 457, 2001.), wherein the size of thespherulites is caracterised by microscopy. The spherulites of reins R9and R10 have a much smaller size than those of the reference resins R1and R3 as can be seen in FIG. 4 a to 4 d. It can be seen that the resinsaccording to the invention have a structure that ressembles that ofpolymers compounded with nucleating additives. As a consequence, theresins according to the invention have an improved impermeability tosolvents as can be seen in FIG. 5. FIG. 5 represents the permeability tofuels expressed in g/day at 40° C. measured on rotomoulded 700 mlbottles having a weight of 150 g and prepared at a PIAT of 190° C. withresins R1, R3, R9 and R10. They also have improved mechanical propertiessuch as for example impact strength and stress crack resistance andimproved optical properties, such as gloss.

The cycle time was measured for rotomoulded articles prepared withresins R1, R3 and R9 at a PIAT of about 190° C. The results aredisplayed in FIG. 6 representing for the three resins the PIATtemperature expressed in ° C. as a function of time expressed inseconds. It can be observed that resin R9, although it has a PIAT higher(by about 8° C.) than that of the other resins, crystallises faster thanresins R1 and R3. Further tests on the cycle time were carried out usinga pressurised mould as described by Crawford (Crawford R. presented at“Advanced seminar in rotational moulding.” held in Minneapolis on Sep.23, 2001.).The results are presented in FIG. 7 and clearly show adramatic reduction in crystallisation time and cycle time for resin R9.

Sintering studies have been carried out following the method describedby Bellehumeur et al. (Bellehumeur C. T., Kontopoulou M., VlachopoulosJ., in Rheol. Acta, 37, 270, 1998.). The resins have been compoundedwith the usual additives recommended for by Ciba. For example resin R9has been additivated with 800 ppm of lrganox® 1010 and 1500 ppm oflrgafos® 168 mixed with zinc stearate to produce resin R9 formulation 2.In another test it has been additivated with 800 ppm of lrganox® 1010and 1500 ppm of Irgafos® P-EPQ mixed with zinc stearate to produce resinR9 formulation 3. These two formulations have an improved coalescencetime as shown in FIG. 8 representing the ratio X/a as a function of timeexpressed in seconds, and wherein X is the sintering neck radius betweenthe two spheres to be sintered and a is the radius of the spheres. Attime zero, X is zero when the two spheres are just touching and as theycoalesce X becomes equal to the radius of the final sphere.

Resins R9 and R10 were ground to powder on state of the art grindingequipment: they exhibited outstanding performances in terms ofproductivity.

Polyethylene resins, prepared with a bis-indenyl metallocene catalystsystem, and having densities in the range of from 0.910 to 0.940 g/cm³and a M12 in the range of from 0.1 to 2 g/ 10 min are not suitable, assuch, for rotomoulding applications. They can however be used as impactmodidfiers in blends for rotational moulding applications. Up to 50 wt %of resins having densities in the range of from 0.910 to 0.940 g/cm³ andM12 in the range of from 0.1 to 2 g/ 10 min can be blended withpolyethylene or with polypropylene for use in rotomoulding, blowmoulding, injection blow mouldind or injection moulding applications.

Resin R9 was also tested under various grinding conditions, using eitherdifferent grinding machines, or using the same grinder to producepowders of different granulometry. The standard pellets were ground intopowders having an average size of about 300 microns and the particlesize distribution analysis was carried out using the method of standardtest ASTM D 1921 and further described in McDaid and Crawford (J. McDaidand R. J. Crawford, in “The grinding of polyethylene for use inrotational moulding.” In Rotation, Spring 1997, 27.). The samples wereproduced on the following machines:

-   -   sample 1: reduction Engeneering machine    -   sample 2: Palman machine    -   samples 3 and 4: Wedco machine with dfferent granulometries    -   samples 5 and 6: Palman HI machine with different        granulometries.

All grindings were carried out at the highest throughput for eachmachine with productions in the range of 800 to 1200 kg/hr. Resin R9 waseasy to grind and grinding reached a steady state after about 30 minutesinstead of the two to three hours necessary for conventional resins. Nostatic electricity problems were observed. The granulometry analysis forthese 6 samples is displayed in FIG. 9 representing the mass fraction ofthe powder expressed in percent as a function of particle size expressedin microns.

The impact of the powders' granulometry on the processability was thenstudied for these same 6 samples, the most important factor beingdensification process resulting from bubble removal. The results aredisplayed in FIG. 10 representing the number of bubbles per mm²remaining in the molten resin as a function of temperature expressed in° C. for all six samples. Quite surprisingly, the resins having thelargest particle size displayed the highest rate of bubble removal as afunction of increasing temperature.

Resins R1, R7, R9, R11 and R12 were tested for micropellets production.Three types of machines were used (Gala, BKG, Black Clawson) and thepelletisation conditions for each machine are summarised in Table VII.TABLE VII Machine 1 Machine 2 Machine 3 Resin R9 R1 R12 R9 R1 R11 R9 R7Micropellets <D_(eg)> (μm) 700 600 650 525 525 470 600 550 Throughput(kg/h) 60 80 60 100 100 100 3 4 Yield (%) 65 95 95 95 95 95 <50 <50Residual water no no No yes yes yes Yes yes Pelletisation Extruder typeTwin screw: 60 mm Twin screw: 75 mm Twin screw: 50.8 mm Melt pump yesyes Yes yes yes yes No no Number of holes 300 300 300 2200 2200 2200 520520 Hole diameter (μm) 400 400 400 300 300 300 350 350 Hole length (mm)3 3 3 — — — 1.5 1.5 Number of knives 18 18 18 6 6 6 6 6 ConfigurationUW¹ UW UW UW UW UW WR² WR Conditions — Melt die T (° C.) 228 236 242 240249 240 250 250 Pressure at die (bar) 130 149 174 163 186 198 1100 1300Knives speed (rpm) 3590 3595 3585 3600 3600 3600 3400 3400 Water T (°C.) 75 74 79 73 71 66 18 18 Water rate (m³/h) 20 20 20 10 10 10 — —Water pressure (bar) — — — — — — 0.68 0.68 Throughput/hole 0.2 0.267 0.20.045 0.045 0.045 0.006 0.006 (kg/h) Shear rate/hole (s-1) 12000 1590012000 6450 6450 6450 470 700 Melt fracture no yes Yes no yes yes No yesUW¹: under waterWR²: water ring

From Table VII it can be concluded that, on all types of machines, resinR9 was converted into micropellets at lower die pressure and at lowermelt die temperature than the other resins tested, and steady stateoperations were established after 30 minutes. In addition R9 did notexhibit any melt fracture even at a very high shear rate of 12000 s-1.

It must also be noticed that high viscosity resins R11 and R12 wereeasily converted into micropellets even though they had a very low meltindex. The size distributions for several resins and machines aredisplayed in FIG. 11 representing the mass fraction of the micropelletsexpressed in percent as a function of particle size expressed inmicrons. The micropellets samples are displayed in FIG. 12 representinga photograph of the micropellets produced on machine 2 for resin R9,under the conditions described in Table VII.

A comparison of granulometry analysis for powders and micropellets isdisplayed in FIG. 13 representing the mass fraction of powders andmicropellets expressed in percent as a function of particle sizeexpressed in microns for resin R7 and showing clearly that the averagesize of the micropellets is substantially larger than that of powders.

The impact of the powders' and micropellets' granulometry on theprocessability was then studied for several samples, by way of thedensification process resulting from bubble removal. The results aredisplayed in FIG. 14 representing the number of bubbles per mm²remaining in the molten resin as a function of temperature expressed in° C. for several powder and micropellet samples of variousgranulometries.

It can be concluded from this figure that the micropellets exhibit abetter bubble removal as a function of temperature than do the groundproducts.

In addition, the high viscosity resin R11 having a melt index M12 of 0.9g/10 min behaved remarkably well, and the limitation in rotationalmoulding to resins having a melt index larger than 2 g/10 min has thusbeen overcome.

Also, resins R11 and R12 have a high molecular weight and thus excellentmechanical properties: they are excellent candidates to replace thecross-linked polyethylene (XLPE) resins generally recommended in thefield of rotational moulding.

1-14. (canceled)
 15. A hollow article produced by rotational molding ofa polyethylene resin and having a wall structure comprising of apolyethylene resin produced by the polymerization of ethylene in thepresence of a metallocene catalyst having a bis-cyclopentadienyl ligandstructure or a bridged bis-indenyl ligand structure.
 16. The article ofclaim 15 wherein said polyethylene resin is produced by polymerizationof ethylene in the presence of a metallocene having a bridgedtetrahydroindenyl ligand.
 17. The article of claim 16 wherein saidmetallocene is ethylene bis (4,5,6,7-tetrahydro-1-indenyl) zirconiumdichloride.
 18. The article of claim 15 wherein said resin is producedby the polymerization of ethylene in the presence of a metallocenehaving a bis(n-butyl-cyclopentadienyl) ligand structure.
 19. The articleof claim 18 wherein said metallocene is bis(n-butyl-cyclopentadienyl)zirconium dichloride.
 20. The article of claim 15 wherein said resin isan ethylene homopolymer.
 21. The article of claim 15 wherein said resinis a copolymer of ethylene and a C₃-C₂₀ alpha olefin.
 22. The article ofclaim 21 wherein said alpha olefin is selected from a group consistingof propylene, butene, hexene, octene and 4-methyl-pentene.
 23. Thearticle of claim 15 wherein said polyethylene resin has a density withinthe range of 0.915-0.95 g/cm³ and a melt index within the range of0.5-30 g/10 min.
 24. The article of claim 23 wherein said polyethyleneresin has a density within the range of 0.925-0.945 g/cm³ and a meltindex within the range of 2.0-20.0 g/10 min.
 25. The article of claim 15wherein said wall structure has a warpage factor which is less than thewarpage factor of the wall structure of a comparative hollow articlearrived at by rotational molding under the same conditions as employedin producing said first recited hollow article of a correspondingpolyethylene resin produced by the polymerization of ethylene in thepresence of a Ziegler-Natta catalyst.
 26. The article of claim 15wherein said wall structure has an elastic modulus which is less thanthe elastic modulus of the wall structure of a comparative hollowarticle arrived at by rotational molding under the same conditions asemployed in producing said first recited hollow article of acorresponding polyethylene resin produced by the polymerization ofethylene in the presence of a Ziegler-Natta catalyst.
 27. The article ofclaim 15 wherein said wall structure has a shrinkage factor which isless than the shrinkage factor of the wall structure of a comparativehollow article arrived at by rotational molding under the sameconditions as employed in producing said first recited hollow article ofa corresponding polyethylene resin produced by the polymerization ofethylene in the presence of a Ziegler-Natta catalyst.
 28. The article ofclaim 15 wherein said wall structure has a permeability which is lessthan the permeability of the wall structure of a comparative hollowarticle arrived at by rotational molding under the same conditions asemployed in producing said first recited hollow article of acorresponding polyethylene resin produced by the polymerization ofethylene in the presence of a Ziegler-Natta catalyst.
 29. The article ofclaim 15 wherein said wall structure has a resistance to degradation bynitric acid which is less than the resistance to degradation by nitricacid of the wall structure of a comparative hollow article arrived at byrotational molding under the same conditions as employed in producingsaid first recited hollow article of a corresponding polyethylene resinproduced by the polymerization of ethylene in the presence of aZiegler-Natta catalyst.
 30. The article of claim 15 wherein the surfaceof said wall structure has a higher gloss than the gloss of the surfaceof the wall structure of a comparative hollow article arrived at byrotational molding under the same conditions as employed in producingsaid first recited hollow article of a corresponding polyethylene resinproduced by the polymerization of ethylene in the presence of aZiegler-Natta catalyst.
 31. The article of claim 15 wherein said wallstructure has a microstructure which is finer than the microstructure ofthe wall structure of a comparative hollow article arrived at byrotational molding under the same conditions as employed in producingsaid first recited hollow article of a corresponding polyethylene resinproduced by the polymerization of ethylene in the presence of aZiegler-Natta catalyst.
 32. The article of claim 15 wherein saidpolyethylene resin has a molecular weight distribution which is lessthan that of a corresponding polyethylene resin produced by thepolymerization of ethylene in the presence of a Ziegler-Natta catalyst.33. The article of claim 15 wherein said wall structure has an elasticmodulus which is less than the elastic modulus of the wall structure ofa comparative hollow article arrived at by rotational molding under thesame conditions as employed in producing said first recited hollowarticle of a corresponding polyethylene resin produced by thepolymerization of ethylene in the presence of a Ziegler-Natta catalyst.